Abstract: In August–October 2021, specimens of one of the most effective and widespread alien mollusk species on a global scale, Dreissena polymorpha (Pallas, 1771), were found for the first time in the Pyshma River on the territory of the West Siberian Plain. The first finding in Siberia was made in a watercourse with a natural thermal regime; the found mollusk individuals were of different size and age groups, which could indirectly indicate successful naturalization of the species.